All-optical signal processing is a key to enabling all-optical networking. All-optical wavelength conversion (AOWC), for example, can be used to perform wavelength routing and contention resolution in transparent optical data networks. As a promising alternative to optical-electrical-optical (OEO) conversion, AOWC can reduce cost and power consumption in high-bit-rate core optical networks.
All-optical format conversion is another functionality needed in all-optical networks composed of multiple modulation formats. For example, all-optical on-off-keying (OOK) to phase-shift-keying (PSK) converters can seamlessly integrate OOK networks and PSK networks. All-optical OOK-to-PSK converters also can be beneficially used in all-optical pattern recognition circuits, as PSK is preferred for passive pattern recognition using all-optical correlators. Because of the constant intensity pattern of PSK formatted signals, PSK is preferable where semiconductor optical amplifiers (SOAs) are used as optical amplifiers.
All-optical logic operations, such as XOR, AND, OR, are an integral part in building all-optical data networks, where packet routing, data buffering, and wavelength conversion are expected to be processed in the optical plane. The XOR operation, for example, is particularly useful for label recognition and swapping, pattern generation, and parity checking. The XOR logic element is a key building block for pseudo-random binary sequence (PRBS) generators and optical half adders, for example.
Integrated semiconductor optical device technology is maturing and much research has been done in optical signal processing using the nonlinear optics in SOAs. However, the pattern dependence of SOAs owing to slow carrier recovery has been a key factor limiting the speed of all-optical logic operation based on semiconductor optical devices. Existing all-optical signal processing methods based on semiconductor optical devices, however, are better suited for OOK than PSK, as the dominant optical nonlinearity, such as cross gain modulation (XGM) or cross phase modulation (XPM), is mediated by intensity modulation and insensitive to the optical phase.
Efficient, all-optical signal processing methods that are applicable to multiple modulation formats, and that have reduced pattern dependence are desirable. For example, because of its superior performance in comparison to OOK, differential phase-shift-keying (DPSK) is increasingly recognized as a promising modulation format for high-bit-rate (e.g., ≧10 Gb/s) optical transmission. Given the importance of DPSK for high-speed optical networks, an efficient method of AOWC for DPSK is highly desired. In contrast to the several methods of AOWC demonstrated for OOK, the few AOWC methods demonstrated for DPSK are based on four-wave mixing (FWM) in optical fibers or in a semiconductor optical amplifier (SOA). Intensity-dependent optical nonlinearities, such as cross gain modulation (XGM) and cross phase modulation (XPM), are usually considered inapplicable for AOWC of DPSK because of their phase insensitivity. FWM is highly polarization sensitive, however, and its conversion efficiency is typically less than the conversion efficiencies that have been achieved with XGM or XPM in SOAs. In addition, FWM in SOA also suffers from high OSNR degradation owing to the relatively high level of amplified spontaneous emission.